1. Field of the Invention
This invention relates to a video signal processing circuit and, more particularly, to a video signal processing circuit for extending the dynamic range of a video signal.
2. Description of the Prior Art
Video signal processing circuits configured to shift the dc level of a luminance signal in order to increase the dynamic range are known. One of such video signal processing circuits is shown in FIG. 1 in which a luminance signal Ys enters through a video input terminal 51, and it is supplied to a dc shifting circuit 52 and a comparator 53. The input luminance signal Ys is compared with a predetermined reference level (hereafter referred to as ref) in the comparator 53. An output signal from the comparator 53 is integrated in a one-field integrating circuit 54 and then supplied to a microcomputer 55. The microcomputer 55 computes an amount of shifting the dc level corresponding to the signal supplied from the one-field integrating circuit 54, and supplies the result of the computation to the dc shifting circuit 52.
When the component of the luminance signal Ys is determined to be higher than the reference level ref in the comparator 53, the integral output from the one-field integrating circuit 54 is lowered. On the basis of this output, the dc level shifting amount to be supplied to the dc shifting circuit 52 from the microcomputer 55 is increased. When the component of the luminance signal Ys is determined to be higher than the reference level ref, the integral output from the one-field integrating circuit 54 is increased. On the basis of this output, the dc shifting amount to be supplied to the dc shifting circuit 52 from the microcomputer 55 is decreased. The luminance signal corrected in dc level by the dc shifting circuit 52 undergoes gamma correction by a gamma correcting circuit 56, and it is then output from a video output terminal 57. For the aforementioned circuit arrangement, a gamma correcting circuit which supplies a high gain when the level of the luminance signal Ys is low can be positively used.
FIGS. 2A, 2B, 3A and 3B show relations between the luminance signal Ys as an input signal to the above-mentioned video signal processing circuit and a luminance signal Ys1 as an output signal. In each of FIGS. 2A, 2B, 3A and 3B, the ordinate refers to the level of the luminance signal, and the abscissa to time.
As shown in FIG. 2A, by shifting the dc level of the luminance signal Ys having a high frequency only in a portion where the luminance level is high, the luminance signal Ys1 shown in FIG. 2B is obtained In FIG. 2B, the high frequency component in the luminance signal Ys1 is shifted in dc level, and undergoes gamma correction by the gamma correcting circuit 56. As a result, a signal having a high contrast is output from the video output terminal 57. At the same time, a dc component included in the luminance signal Ys is also dc-shifted together with the high frequency component. Therefore, the luminance signal is lowered to the black level shown by a broken line. As a result, the luminance level of the luminance signal in the dc component becomes the black level, and the signal is output as indicating "black" from the video output terminal 57. Therefore, when the signal is actually reproduced on a monitor, the entirety of the monitor becomes dark.
Further, as shown in FIG. 3A, by shifting the dc level of the luminance signal Ys having a high frequency only in a portion where the luminance level is low, the luminance signal Ys1 as shown in FIG. 3B is obtained. In FIG. 3B, the high frequency component in the luminance signal Ys1 is shifted in dc level, and undergoes gamma correction by the gamma correcting circuit 56. As a result, a signal of a high contrast is output from the video output terminal 57. Consequently, part of the high frequency component overlaps the black level shown by a broken line; however, when it is actually reproduced on a monitor, it does not matter from the visual viewpoint.
If such dc shifting is executed when the video signal shown in FIG. 2A is input, the signal having a low luminance level is lowered to the black level at the same time when the dynamic range of the high frequency component is extended. As a result, it is possible that the entirety of the monitor becomes dark. Therefore, the dc shifting amount for the luminance signal Ys is normally limited small. Although the modulated component in the luminance signal Ys1 shown in FIG. 3B should have a wider dynamic range, the dc shifting amount is maintained small in the prior art video signal processing circuit. Therefore, when the luminance signal shown in FIG. 3A is input, a sufficiently large dynamic range cannot be obtained.